A 2-year-old girl was hospitalized in critical condition and was not conscious or breathing when firefighters found her, officials said. Her grandfather was also hospitalized. Jane Yamamoto reports from West LA for NBC4 News at 11 p.m. Friday, Oct. 18, 2013. (Published Saturday, Oct. 19, 2013)

A fire in a West LA apartment high-rise injured at least five people, including a 2-year-old girl, and displaced residents of 51 units Friday.

The blaze began in a unit on the 11th floor, and sent thick smoke into the building's upper floors. An estimated 100 to 150 people were unable to return to their homes, and some were receiving assistance from the American Red Cross, according to Los Angeles Fire officials.

Aerial video showed firefighters treating an adult and the child on the roof of the complex after the fire was declared a knockdown.

The girl and the man, who was identified as her grandfather, were found lost on the 18th floor and brought to the roof for treatment before being transported to a hospital, according to the Los Angeles Fire Department. The girl, who was reportedly not breathing when firefighters found her, remained hospitalized in critical condition.

One pet dog also died in the fire.

Details regarding the injuries to the adult victims were unclear, but firefighters said they are not considered life threatening.

"We're encountering a handful of residents in and outside the building with minor to moderate respiratory complaints," the LAFD said in an alert issued after the knockdown. "As such people come forth, our patient tally has the potential to climb."

Three firefighters were treated and released from Grossman Burn Center for non-life-threatening burns. However, they remained off-duty after their release, according to Fire Department officials.

The fire broke out just before noon inside an 11th-floor unit at 11740 Wilshire Blvd, the 25-floor Barrington Plaza. The fire was isolated to one two-bedroom unit -- built in 1961, it was not equipped with a sprinkler system -- on the 11th floor and knocked down in about one hour.

The fire set off alarms inside the building. Some residents heard them, but others did not, fire officials said.

Firefighters responded to several reports of individuals stranded in stairwells. Residents in some floors above the burning unit who did not respond to alarms and leave the building might have been exposed to smoke, said Brian Humphrey, of the Los Angeles Fire Department.

"In a fire like this, the most important thing is to know your exits," said Humphrey.

Firefighters with heavy equipment strapped to their backs climbed stairs to reach the burning unit. An LAFD helicopter also responded to the site and landed on a helipad at a nearby high-rise building.

Some residents on floors below the fire were asked to shelter in place, said Armando Hogan, of the LA Fire Department. The strategy keeps stairwells clear so firefighters have a path to the fire.

"We have to hike up stairs to get to that floor with equipment that weighs upwards of 100 pounds," Hogan said.

Details regarding a cause were not immediately available.

Wilshire Boulevard was closed from San Vicente Boulevard to Bundy Drive. Several firefighting vehicles were parked along the busy route west of the 405 Freeway.

More than 180 firefighters responded to the location from stations throughout Los Angeles.